1. Field of the Invention
The present invention relates to a light scanning apparatus for reading or developing an image, and more particularly to a light scanning apparatus in which a light source of the light scanning apparatus is continuously lit during operation of the apparatus.
2. Description of the Related Art
As is well known, when an ultrasonic wave of a particular frequency is applied to an acousto-optic modulating element, light input thereto in a specific direction with respect to the propagation direction of the ultrasonic wave is deflected, due to an effect called Bragg Reflection. Thus, in a high speed printer employing a gas laser as its light source, for example, an acousto-optic modulating element has been employed to modulate, i.e. switch, the continuously output laser light. Light emitting diodes (LEDs), which can be directly modulated, have also been employed as a light source for high speed printers. However, it is difficult to match the light spectrum of LEDs with that of a photosensitive drum of the printer and LEDs have a relatively short life. Gas lasers are therefore still employed as light sources.
A block diagram of a prior art laser printer system using an acousto-optic light-modulating element is shown in FIG. 1. A laser light produced by a light source 1 (e.g., gas laser) is focused by a focusing lens system 2 onto a crystal of an acousto-optic light-modulating element 4 to form a fine light spot thereon. On receiving an enabling signal from a control circuit 10, a modulator driver 3 outputs a modulating signal of approximately 200 MHz to the acousto-optic light-modulating element 4. The acousto-optic light-modulating element 4 deflects the light input thereto by an angle of 23 m radians, through a slit 20 in a plate 19 and onto an expander lens system 5.
When no modulating signal is applied to the acousto-optic light-modulating element 4 from the modulator driver 3, the acousto-optic light-modulating element 4 does not deflect the light through slit 20. Instead, the light is blocked by plate 19 and thus the light from the focusing system 2 never reaches the expanding lens system 5.
In FIG. 1, the deflected light is designated by arrow D and the not-deflected light is designated by arrow ND. The deflected light D reaches the expanding lens 5 through slit 20; however, the not-deflected light ND is directed onto the plate 19 and is blocked from reaching expander lens system 5.
The deflected light D reaching the expander lens system 5 is expanded to form a parallel light beam which is directed to rotating polygonal mirrors 6. Before reaching a photosensitive drum 8, the light is deflected by the rotating polygonal mirrors 6 so as to scan the beam detector 9. The light deflected by the mirrors 6 is focused by an f..theta.lens 7. The lens 7 directs a finely focused light spot onto the photosensitive drum 8. Beam detector 9 located optically next to the photo-sensitive drum 8 detects the light scanning a single frame (i.e., a single light scan from one end of drum 8 to the other end of drum 8) on the drum 8, prior to the scanning light beginning to actually scan drum 8. Upon detecting the light, the beam detector 9 transmits a detection signal BD (beam detection) to control circuit 10. In response to receiving the detection signal BD, the control circuit 10 allows a VIDEO signal to be transmitted sequentially to the modulator driver 3. The VIDEO signal acts as a modulator driver signal. When the signal is a "1" level, the modulator driver 3 causes the acousto-optic modulating element 4 to deflect the input light through the slit 20.
Referring to FIGS. 2 and 3, the structure and operation of the acousto-optic light-modulating element 4 are explained below. FIG. 2 schematically illustrates the structure of the acousto-optic light-modulating element 4. The acousto-optic light-modulating element 4 includes a crystal portion 4' comprising, for example, lead molybdate (PbMo0.sub.4) or tellurium dioxide (Te0.sub.2) and an electro-acoustic transducer 4" comprising, for example, lithium niobate (LiNb0.sub.3). FIG. 3 illustrates waveforms of the modulating signal "a" provided by the modulator driver 3 and the deflected light "c" output from the acousto-optic light-modulating element 4. When no modulating signal "a" is applied to the transducer 4", the laser light beam "b" travels straight through the crystal portion 4' of the element 4 and is output as shown by dotted lines L5 and L6. When a modulating signal "a" is applied to the transducer 4", an ultrasonic wave L7 of approximately 200 MHz generated therein propagates through the element 4 approximately orthogonal to the direction of the input light beam "b". As soon as the front end of the ultrasonic wave reaches the upper edge L1 of the input light beam "b", the upper edge of the light is deflected along line L3 in the direction "c". It takes a time period t.sub.r (FIG. 3) for the ultrasonic wave L7 to reach the lower edge L2. Therefore, the rise and fall of the waveform of the deflected light pulse, i.e., the output of the acousto-optic light-modulating element 4 is delayed and deformed as shown by "c" in FIG. 3. The delay time period, i.e., the rise time period t.sub.r is determined by:
t.sub.r =0.66d/v
where d is the diameter of the input light beam "b", and v is the propagation velocity of the ultrasonic wave L7 in the crystal portion 4' of the acousto-optic modulating element.
In order to achieve a less deformed modulated light output, i.e. one having fast rising and falling edges, the diameter of the light spot is required to be as small as possible. This is because the delay time period t.sub.r is shorter for a smaller diameter d of the input light beam b. If the light output pulse is deformed, a dot to be printed may be missed. The recent trend for speeding up the light scanning apparatus and achieving higher resolution requires that the light spot size be reduced from about 100 .mu.m in diameter, to as fine as 30 .mu.m. The smaller spot size means that there is a larger energy density of the focused light; the energy density varies inversely with the square of the spot diameter. However, the allowable light input to the acousto-optic light-modulating element is limited by the energy density of the light input thereto as well as by the accumulated duration of the light input. As a solution to this problem, a mechanical shutter may be provided between the light focusing system 2 and the acousto-optic light-modulating element 4; however, the shutter speed is too slow and the durability of the shutter is too short to be efficient. Moreover, a solenoid used to drive the mechanical shutter generates electrical noise which disturbs electronic circuits in the vicinity of the solenoid.